Constraining Decadal-Scale Erosion and Delivery of Post-Wildfire Debris Flow Deposits

Debris flows generated from wildfire pose significant risk to increased sedimentation to instream river networks, degradation of water quality, and accumulated sediment behind downstream reservoirs. Despite an abundance of research investigating the probability of debris flow generation, the constraints on deposit initial volume, and most recently the initial grain size distribution, there remains a significant knowledge gap in understanding the temporal scale at which debris flows supply sediment to river networks after deposition. To provide reliable estimates of sediment delivery from debris flow deposits over time, two important metrics must be constrained: 1) how does sediment delivery to river networks from post-wildfire debris flow deposits change over time, and 2) which local, reach-scale hydrogeomorphic features influence the relative magnitude of debris flow sediment delivery over time? With a combination of remote analysis and fieldwork of 58 identified debris flows in the state of Utah, we have compiled a dataset representing volume loss from debris flow deposits through time. Three erosional processes are responsible for removing debris flow sediment over time. The axial river removes the largest magnitude of sediment from post-wildfire debris flow deposits due to fluvial processes at the toe of the deposit. We additionally observed channel incision on the deposit surface via tributary channels emerging from the generating tributary catchment. Lastly, we inferred a third process, surface deflation, that results in volume loss from the debris flow deposit without any change to the deposit area. We found that initial volume loss is heavily driven by channel confinement and long-term erosion by the axial river can be modeled as an exponential decay function. Additionally, we proposed a new metric, the Debris Flow Delivery Potential, as a method to estimate the probability that a debris flow deposit would be initially delivered into the axial river. The framework presented here is aimed to improve long-term hazard assessment models regarding post-wildfire debris flow mitigation and planning.